Each of an electroluminescent element and a light-emitting diode having a light-emitting layer interposed between electrodes to obtain electroluminescence has been vigorously developed not only for application to a display device but also for application to various types of light sources such as a planar lighting, a light source for optical fiber, a backlight unit for a liquid crystal display, and a backlight unit for a liquid crystal projector.
In particular, an organic electroluminescent element is excellent in light-emitting efficiency, low-voltage drive, weight, and cost and has attracted attention in recent years. Device structure, material, a driving method, a manufacturing method, and the like have been developed in order to obtain light-emitting efficiency comparable to fluorescent lamps for application to lighting.
However, in an inter-solid light-emitting element (e.g., organic electroluminescent element) designed to allow light to be extracted from the light-emitting layer per se, if the light has an angle not lower than a critical angle, the light is totally reflected and is confined in the inside and is consequently lost as guided light. The critical angle depends on refractive indexes of the light-emitting layer and an emitting medium.
According to the Snell's law, a light-extracting efficiency η that is an efficiency of extraction of light to an outside is approximated with an equation of η=1/(2n2) where n denotes the refractive index of the light-emitting layer. Provided that the refractive index of the light-emitting layer is 1.7 which is a representative value among refractive indexes of organic compounds, η is about 17% and not less than 80% of the emission light is lost as guided light, that is, as lost-light towards a side direction of the element.
In order to extract such guided light, a region for disordering reflective and refractive angles and for disturbing the Snell's law is required between an emission face and the light-emitting layer so as to change a transmission angle of light that is otherwise totally reflected as guided light. Alternatively, the emission light is required to have a light-harvesting property.
In typical organic electroluminescent elements described in Patent Literatures 1 and 2, a thickness of an electron transporting light-emitting layer is in a range of several tens nm to one hundred and several tens nm and has the same orders of visible light wavelengths. Therefore, light waves (light) emitted consequently to the outside interfere with each other destructively or constructively depending on a distance d between a light-emitting region and a reflective electrode. Although light emitted along a normal direction of the element is only shown in FIG. 13 in Patent Literature 1, light emitted along an oblique direction is also present actually. Conditions for interference vary depending on the angle of the light as well as the distance d and an emission wavelength λ. Therefore, a case where light waves emitted along the normal direction interfere with each other constructively and light waves emitted along a wide-angle direction interfere with each other destructively may occur, or the opposite case may occur. In other words, luminance of the emission light varies depending on the viewing angle.
Patent Literature 1 and Patent Literature 2 discloses a method for improving light-emitting efficiency of an organic electroluminescent element having a light diffusion structure, by designing to satisfy the following formula (1) in luminance distribution of the light-extracting face of the organic electroluminescent element before formation of the aforementioned structure for extracting guided light, and subsequently by forming the light diffusion structure or the like on the light-extracting face of this element.Front luminance<(luminance for an angle in a range of 50° to 70°)   (1)
In this invention, in a state that no light diffusion layer is formed, light waves along the normal direction are designed to interfere destructively but guided light waves confined in the inside of the element are designed to interfere constructively. In this way, the invention indicates that, in order to improve the light-emitting efficiency of the organic electroluminescent element, the light diffusion layer should be formed on the organic electroluminescent element having a basic structure designed to cause constructive interference in light (a wide angle component light), which is usually confined in the element as guided light, so as to amplify the guided light, which consists a greater part of light, rather than on a basic structure designed to cause constructive interference in light along the normal direction that can be emitted to the outside.
Note that the formula (1) may be converted into the following formula (2).(An average luminance for an angle in a range of 50° to 70°)/a frontal luminance>1   (2)
Moreover, Patent Literature 2 discloses that, in an organic electroluminescent element having light-emitting layers of different colors, a distance between a cathode and a light-emitting layer, which has poor efficiency, is optimized. For application of the organic electroluminescent element to lighting, it is preferable not only that light-emitting efficiency is high but also that viewing angle dependence of an emission color is low. Uniform viewing angle dependences of the emission intensities of emission colors ideally provide no viewing angle dependence of the white light-emitting organic electroluminescent. However, not uniform viewing angle dependences of the emission intensities of emission colors are required for adjusting the light-emitting efficiencies of emission colors by using an effect of optical interference as shown in Patent Literature 2.
As described in Patent Literature 3, it is generally known that the viewing angle dependence of the emission color is suppressed by forming a light scattering layer having light scattering property on the light-extracting face of the organic electroluminescent element. Therefore, the viewing angle dependence is furthermore reduced by forming the light diffusion layer in the organic luminescent element having the viewing angle dependence of the emission color that has been already reduced without the light diffusion layer.